Needleless injectors are used as an alternative to conventional hypodermic injectors to deliver medicaments through the patient's skin into the underlying tissues. Such injectors use a high pressure piston pump to dispense a jet of liquid drug with sufficient force to penetrate the skin, and thereafter deposit the drug into the dermal, subcutaneous or muscular tissues.
The drug is dispensed from a cylindrical chamber, having a fine orifice at one end through which the drug is discharged. A piston is slidingly and sealingly located in the chamber, and the drug is contained within the space between the orifice and piston. To make an injection, the orifice is placed on the skin, and by operating a release mechanism, the piston is acted upon by a force which may be derived from a spring, pressurised gas or chemical reaction.
The capsule may be filled by the user, or may be prefilled and pre-assembled to an actuator. In the latter case particularly, the materials from which the capsule and piston are constructed must be inert to the drug--i.e. they must not react with the drug chemically, nor physically, and must not contain harmful extractives that might contaminate the drug. The choice of materials is small: borosilicate glass is the most favoured capsule material when drugs must be stored for more than a few hours. If an alternative material is selected for the capsule, years of testing must be done to validate that material, whereas borosilicate glass has a known compatibility with most drugs.
During the injection, the pressure generated in the capsule is at least 100 bars, and it is preferable, in order to avoid leakage during injection, that the orifice is integral with the cylindrical chamber. Furthermore, the form and dimension of the orifice is critical to the injection performance, and for repeatable results these features should be made to close tolerances. However, glass is a difficult material to mould and maintain such close tolerances over many millions of components. One traditional method is to work the heated and softened end of a glass tube on a lathe, and by applying a shaping wheel or paddle, to close up one end onto a mandrel to form the orifice. This is a relatively crude method, and the only parameters that may be controlled accurately are the orifice diameter and the diameter of the surrounding glass: the length and entry profile of the orifice are left to chance because the process shapes only the outside of the tube and the orifice diameter. An alternative process is moulding, whereby a hot "gob" of molten glass is moulded in a die. This process is suitable for large components, but needleless injector capsules are seldom larger than 1 ml capacity, and such a small gob of glass loses its heat rapidly and is difficult to mould. Also the surface finish inside a moulded tube is not smooth enough for this application, nor is the bore parallel. Drawn tubing, which has an excellent surface finish and form, is the preferred starting material, but current working methods, as described, do not provide control of both inside and outside dimensions.
Conventional glass hypodermic syringes are made on automatic lathes from glass by working heat-softened tube, as previously described. Low cost disposable glass syringes are generally made with the hollow needle glued into a precisely formed hole in one end of the syringe body. The manufacturing process is relatively primitive, with low production rates and high reject rates.